In the United States, there is fifteen times as much recoverable coal as recoverable oil and natural gas combined. Coal, therefore, should be the primary fuel for large stationary and mobile combustion installations and for production of process heat. Not only should America's energy needs increasingly be met by coal, but coal could also meet the needs of other industrialized and developing countries. Coal could be America's answer to the "energy" balance of trade deficit. However, such is not presently the case.
Although 200 billion tons of economically recoverable coal, and an undetermined amount of coal which is not presently economically recoverable, are present in the United States (representing more than 70% of domestic fossil energy resources), coal currently supplies less than 20% of all of our energy production. A number of factors have combined to create this disparity. Even with the abundance of coal energy, it has not heretofore been competitive with, nor as easily utilized as, other fossil fuels such as oil, natural gas and the like.
One very effective use of our coal resources is in stationary plants producing electricity or process heat. Stationary power conversion facilities do not require high performance liquid and gaseous hydrocarbons. These fuels are better utilized in transportation and certain residential/commercial uses. However, the use of coal in stationary power facilities requires that either the solid be transported to the power facility or the power plant be constructed at the mine site for "mine mouth" utilization of the coal. Utilization at the mine site is not always an efficient method for producing electricity because of environmental problems, and electrical transmission losses. Yet, production of electrical energy at other than the mine site requires that the solid coal be transported to the power plant. Coal is currently shipped by rail in unit trains but the current rail capacity may be inadequate to move the tonnage required to replace existing use of more costly and scarce liquid and gaeous fuels. Moreover, the required handling of coal as a solid fuel is cumbersome, wasteful and expensive.
The greatest deterrent to full utilization, domestic and foreign, of the United States' coal resource is the nature of coal itself. First, raw coal is not a uniform combustion product. Second, as a solid it is difficult to handle and expensive to transport. Third, it contains organic sulfur and nitrogen, which, upon combustion, produce air pollutants which have been associated with acid rain. Fourth, it contains ash which, upon combustion produces pollutants and slag. In addition to the above problems, the majority of the energy transportation and combustion systems in this country revolve around oil and natural gas which are relatively uniform, pipeline tranportable liquid and gaseous fuels. The coal transportation and quality problems are compounded by the fact that, although coal reserves are distributed throughout the U.S., coal from different reserves has a wide range of characteristics. Coals, even of the same rank, have different compositions. This limits the interchangeability of coal in combustion systems and thus increases expense and reudces markets. For example, intermountain Western coal, while low in sulfur, is also generally low in BTU per unit weight and has a high water content. Each type of coal requires different pollution control equipment and a specific boiler system. Coal of one region (or even of a particular mine) cannot be efficiently combusted in boilers designed for coal from another source. Therefore, coal is not as uniform a fuel as is, for example, #6 fuel oil.
The inefficient and expensive handling, tranportation and storage of the solid material has made the conversion of oil-fired system to coal less economically attractive. Liquids are much more easily handled, transported, stored and fired into boilers. Because of this nation's dependence on oil and natural gas, existing fuel transportation systems in the U.S., from pipelines to ocean-going tankers, are designed for liquids and gases.
Various methods, for the most part not economically viable, have been proposed for converting coal to synthetic liquid or gaseous fuels. Recently developed process technology permits the conversion of coal to synthetic liquid or gaseous fuels at the mine site. While this "synfuel" is more easily transported than coal, the conversion process is capital intensive and requires a great deal of water. The process is also very energy intensive in that essentially every carbon atom in the coal matrix is converted to a hydrocarbon. Despite the high processing costs, the resultant synfuel, like crude oil derived fuels, is valuable as a transportation fuel.
Methods for creating coal slurries or mixtures which facilitate liquid transport and fluidic firing into boiler systems have been proposed but not have been completely successful. To produce a slurry, raw coal is ground, sized, slurried with water or other liquid, and stabilized. The goal is to obtain a product which handles like a liquid, not only facilitating the transportation step itself, but also reducing labor costs and eliminating the many other handling problems of solids and reducing the capital costs required to convert oil-fired systems to use solid coal.
Coal slurries are comprised of ground coal particles which have jagged, nonsymmetrical shapes due to fracturing along crystal faces. This configuration not only is abrasive to conduit systems but also adversely affects the loading limits and flow characteristics of any resultant slurry. Since coal is the main fuel constitutent in such slurries, furnace and stack modifications are still required in order to burn coals from different regions. Non-aqueous liquids used for slurrying (including alcohol) tend to solubilize impurities in the coal. These impurities tend to combine with the liquid medium and form polymerized materials which "varnish" pipelines and alter rheology characteristics.
Previous coal slurries have required special pipelines and pumping equipment. Aqueous coal slurries have additional drawbacks: (1) The water which is necessary to slurry coal is in short supply for coal reserves in the intermountain West. (2) Water must be removed from the slurry and the coal must be dried prior to introduction of the fuel into a furnace or boiler to avoid incurring a substantial heat penalty. (Derating of the boiler) (3) Dewatering and disposal of the slurry water creates a pollution problem.
Liquids other than water, such as alcohol, may be used as the slurrying liquid but are expensive and usually require water for manufacture. In addition to being abrasive, coal slurries tend to settle upon standing, thereby causing flow problems in pipelines and ballast problems aboard ships.
While coal/water slurries and coal/alcohol slurries require substantial system modification in order to be fired in existing oil-fired combustion systems, coal/oil mixtures ("COM") are able to be burned in existing coal-fired furnaces, boilers and process heat generators without substantial equipment modification. COMs, which comprise a pulverized, comminuted or ground coal admixed with oil, may contain various additives to, for example, increase the wetability of the coal, stabilize the mixture, etc. This fuel mixture, while capable of being transmitted by pipeline, requires special handling and pumping equipment. These COMs have received extensive attention in the past decade but they are not new. U.S. Pat. No. 219,181, issued Feb. 24, 1879 to Smith, H. R. and Munsell, H. M. discloses the basic coal/oil mixture and their use. COMs, while generally having a higher BTU content per unit volume than either coal or oil alone, have serious drawbacks. First, the oil used as the slurry medium draws from the U.S. domestic or foreign supply of crude oil; therefore, it only partially cuts down on this country's foreign oil dependence and reduces our balance of trade deficit. Second, there are severe restrictions on the export of oil even as a coal/oil mixture, thus there is a limited foreign market. Third, crude oil is expensive and, with the additional slurrying expense, the cost savings to an oil-fired system are marginal. Finally, these COMs have all the inherent drawbacks of coal-containing slurries.
Although coal/water slurries have not been altogether successful and require substantial amounts of water, which requirement is not easily met in the intermountain west, nevertheless, highly loaded water slurries without the inherent drawbacks previously discussed would have viability in non-arid regions. This would be especially true if the physical structure of the carbonaceous material were conducive to transport in a slurry and if such material were a uniform combustion product sufficiently high in BTU to offset the heat penalty experienced during combustion of the aqueous slurry.
Thus it would be highly advantageous to have a fluidic fuel system which is easily and efficiently prepared from coal and which would be (a) transportable using certain existing pipeline, tank car and tankership systems, (b) burnable either directly as a substitute for oil in substantially all existing oil-fired combustion systems or separable at the destination to provide a burnable carbonaceous material, (c) a uniform combustion product regardless of the region from which the coal is obtained, (d) high in BTU content per unit volume, (e) low in ash, sulfur and nitrogen, (f) high in solid loading and stability, (g) free of polluting process by-products which would have to be disposed of at the production site and (h) produced as part of a process which also yields a high quality liquid organic material which is pipeline transportable and can be used as a liquid fuel or as a quality feedstock material.